1. Field of the Invention
This invention relates generally to control circuits for determining the conductive or non-conductive states of power switching devices. A more particular application of this invention pertains to transformer-coupled base drive control circuits for power switching transistors such as those used in the inverter sections of DC/DC and DC/AC voltage converters.
2. Description of the Prior Art
In converters utilizing transistors as the switching devices in the inverter section, a problem frequently encountered is that of providing an efficient, or hard, turn-off base drive over a wide range of operating conditions. To switch a transistor from the conductive, or saturated state, to a non-conductive, or cut-off state, requires removal of the excess stored base charge in the switching device. Hence, the switching turn-off time is usually a function of both collector current magnitude and duration. The time required to remove the excess base charge is commonly defined as the storage time. At the end of the storage time, the transistor collector current decreases to zero in an interval defined as the fall time.
It is common in regulated coverter circuits for the inverter switching devices to be required to operate at high peak collector currents for very short conduction times. Such a situation occurs, for example, where a substantially short-circuited condition appears at the converter output. Under such a condition, the inverter transistors must supply a peak current, equal to or greater than that current experienced at rated load and output voltage, for very narrow pulse widths. Generally, the commonly used transformer-coupled base drive arrangements of the prior art cannot provide the necessary reverse base current and voltage for efficient turn-off under such very short conduction time operating conditions. Hence, switching losses increase significantly, and the transformer-coupled turn-off control circuitry loses the ability to operate properly at the end of such narrow pulse conduction widths.
The problem may be further explained by noting the general method of operation of transformer-coupled base drive circuits. In prior art circuits, the power transistor turn-off is typically controlled by an arrangement in the transformer primary path providing turn-off potentials which are dependent upon the conduction pulse width of the power transistor. Specifically, turn-off is normally initiated by inducing a reverse current from the transformer primary winding to a secondary winding connected across the base-emitter junction of the power switching device. One such improved circuit for initiating turn-on and turn-off of a power switching device is disclosed in a co-pending application assigned to the same assignee by K. A. Wallace, Ser. No. 581,914 filed May 29, 1975 entitled "Drive Circuit For Power Switching Devices". Wallace discloses means for providing improved turn-off potentials which provide reverse sweep-out current and transformer core flux reset action, and reverse bias for a more stable device non-conduction state.
The amplitude of the reverse base-emitter voltage during the non-conduction interval is determined by the turn-off and transformer core reset circuitry connected to the primary winding. If, as in prior art circuitry, the output impedance of the base drive arrangement is fixed, the circuit's ability to supply reverse turn-off current is reduced in proportion to a reduction in the reverse voltage available at the initiation of turn-off. For long conduction pulse widths, the reverse voltage may be sufficiently large to provide efficient turn-off. However, as the conduction pulse width decreases, the available reverse potential correspondingly decreases. This has been found true even for improved circuits such as that disclosed in the co-pending application by Wallace.
The forward base-emitter voltage and pulse width during conduction determine the positive volt-time integral, or flux change, in the core of the current coupling transformer. To prevent DC magnetization of the transformer core, the negative volt-time integral established during non-conduction must be equal to the positive volt-time integral. Hence, the condition of a narrow forward base-emitter voltage pulse width results in a long turn-off interval, thereby causing a low available reverse voltage at the initiation of turn-off.